It appears the 49ers will attempt to end the first two-game losing streak of the Jim Harbaughera without two All-Pro defensive players.

In addition to outside linebacker Aldon Smith, who will begin treatment this week for what's believed to be a substance-abuse problem, San Francisco could be without inside linebacker Patrick Willis.

Willis left late in the third quarter of Sunday's 27-7 loss to the Colts with a groin injury when he attempted to tackle Colts wide receiver Reggie Wayneon a 25-yard pass play. Willis walked gingerly to the sideline, and his day was soon declared over when an equipment manager took his helmet. After walking slowly to his locker, Willis declined an interview request after the game.

The odds of Willis playing in Week 4 are lessened because the 49ers have just three days before they visit the Rams in St. Louis on Thursday night. If Willis misses just the fifth start of his six-year career, Michael Wilhoite, 27, would make his first career start in his place.

Without Willis on the field, Colts running back Ahmad Bradshawhad 11 carries for 62 yards and scored on a one-yard run in the fourth quarter.

Asomugha leaves early: No. 3 cornerback Nnamdi Asomughaappeared to sustain a left-ankle injury when he tackled Bradshaw after a 27-yard run in the fourth quarter. Asomugha, who did not return to the game, would be replaced by Tramaine Brockif he can't play Thursday night.

Briefly: The 49ers have been outscored 31-0 in the fourth quarter of their past two games. ... Running back Frank Goremoved into second place in franchise history for most receiving yards by a running back with 2,687. Roger Craig had 4,442 career receiving yards.